For environmental protection, there has been a demand for compact batteries with high output density as the power source for vehicles and cellular phones. Specifically, a lithium ion battery, which includes an active material having high output density, has been drawing attention.
The application to the power source for vehicles requires a plurality of lithium ion batteries that are connected in series to increase the output power. However, batteries connected through connection members have a problem of power reduction due to electric resistance of the connection members. In addition, the connection member has disadvantageous effects on the space and weight of batteries.
In order to solve the problems, a bipolar battery has been developed. The bipolar battery includes a current collector having both faces on which a positive electrode active material and a negative electrode active material are provided.
Due to the structure of the bipolar battery, a positive electrode active material layer-forming face of the current collector is required to be stable in an equilibrium potential environment between the positive electrode active material and a lithium ion and the opposite side, that is, the negative electrode active material layer-forming face of the current collector is required to be stable in an equilibrium potential environment between the negative electrode active material and an lithium ion.
For example, Patent Document 1 discloses using metal foil as a current collector. In the case of a current collector using metal foil, a current collector selecting a metal foil such as a SUS foil is stable in equilibrium potential environments in a positive electrode and a negative electrode, but such a current collector increases the weight of a battery, and this limits the improvement in output density.
In contrast, as a current collector having light weight, which should improve the output density, for example, Patent Document 2 discloses a current collector containing a polymer material and Patent Document 3 discloses a current collector including two or more layers in which at least one layer contains a polymer material or a ceramic material.